A punch unit of this type is known from U.S. Pat. No. 5,131,303. In this structure, the back end of the punch, which is provided with an exterior screw thread, is screwed into a threaded bore at the front end of the punch driver. The threaded section at the back end of the punch is provided with four axial, longitudinal grooves, and an open spring washer, whose one end is radially bent inward and projects through a radial hole into one of the axial grooves of the punch, is seated on the exterior circumference of the front end of the punch driver. In the assembled state the guide bushing is frictionally connected via an O-ring with a washer seated on the punch driver underneath the pressure spring and prevents the spring washer from being able to yield radially outward. In this way the punch and the punch driver are directly connected with each other by the spring washer without being able to rotate.
When the punch is to be reground, the punch driver can be pulled away from the guide bushing toward the rear, in the course of which the frictional connection at the O-ring is released. After the spring washer has left the guide bushing, the threaded connection between the punch and the punch driver can be released by relative rotation. Because the radially inward bent end of the open spring washer has a point and the lateral walls of the grooves are inclined, the free end of the spring washer is pushed out of the groove in which it had been engaged, when the punch is rotated in relation to the punch driver and, with continued relative rotation, then engages the next groove. Knowing the thread pitch, it is also known which change in the total length of the punch and punch driver corresponds to the rotational angle between two grooves. Because of this it is possible in a very simple way to reset the total length of the punch and punch driver following the regrinding of the punch. As soon as the guide bushing has subsequently again been pushed over the open spring washer and has been frictionally connected with the punch driver via the O-ring, the screw connection between the punch and the punch driver is again dependably blocked against relative rotation because the guide bushing does not permit the open spring washer to widen radially, so that therefore its radially inwardly bent end no longer can leave the radial groove with which it is in engagement at that time.
Although the known punch unit assures a simple setting of the total length of the punch and punch driver, it suffers from the essential disadvantage that the grooves intended for securing against relative rotation weaken the screw thread through which the large punching forces are transmitted to the punch. This is of particular disadvantage in connection with punches having a relatively small cross section, as finer adjustment is required with decreasing cross section, because it is then necessary to have correspondingly more grooves at the circumference.
Furthermore, a punch unit of the type mentioned at the outset is on the market, wherein an upper and a lower punch driver element are connected by means of an adjustable screw thread, which can be secured in the respectively set relative position by means of two long safety screws, which can be unscrewed upwardly. A change of the total length of the punch driver is a very laborious process in this construction.
In another, also commercially available punch unit of this type, the upper and lower punch driver elements are secured by means of a central, axially displaceable bushing with teeth on the outside, which is maintained in an engagement secure against relative rotation with both elements of the punch driver by the action of a spring and which is disengaged from the upper punch driver element by an axial push against the spring force from above. However, in actual use it is very difficult to press down the thin-walled bushing, which is provided with an axial threaded bore, manually by means of a finger inserted into the screw thread, while simultaneously turning the upper punch driver element by the required amount.